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Solar Energy Conversion
Using Platinum Group Metal
Co- ordination Complexes
A REVIEW OF CURRENT RESEARCH
By D. M. Watkins
Group Research Centre, Johnson Matthey and Co Limited
Co-ordination complexes of ruthenium and rhodium have been shown to
have potential applications i n solar energy conversion. When illuminated
with visible light on a laboratory scale these complexes catalyse photolysis
reactions or exhibit photogalvanic effects to produce hydrogen or electricity.
These processes and the problems involved i n their immediate scale-up to
practical solar energy conversion devices are discussed i n this paper.
Ever since farmers first began to harvest the
products of photosynthesis, man has been
indirectly utilising the energy received on
earth from the sun. More direct applications
of solar radiation in mirrors and reflectors and
for the performance of physical work such as
the evaporation of brine to yield valuable salts
are equally as old. However, because of the
lack of suitable technology and the ease of
access to alternative energy sources which are
independent of climate and weather conditions, the harvesting of solar energy has been
sadly neglected almost to the present day.
Only over the past decade or two has the
growing concern about the rapid depletion
of the world’s natural non-renewable resources stimulated extensive research into the
direct conversion of solar radiation into
alternative forms of energy. These conversion processes may be broadly classified ( I ) as :
(a) thermal conversion, for example solar
furnaces,
(b) direct electrical generation, including
photovoltaic or photogalvanic devices,
(c) photochemical processes, such as photosynthesis or photolysis of water to form
hydrogen and oxygen.
Platinum Metals Rev., 1978, 22, (4), 118-125
This article considers the potential application of co-ordination complexes of ruthenium
and rhodium in the photogeneration of
hydrogen gas or electricity.
Electrically
generated power needs to be stored for use
when the sun is not available. At present, the
only practical solution is to use conventional
storage batteries which have to meet stringent
specifications including good cycle lives, low
open-circuit losses, good charging efficiencies
and low maintenance requirements.
An
attractive long-term solution, however, would
be to store electrical energy as hydrogen
obtained from the electrolysis of water.
Hydrogen produced in this way, or by the
direct photolysis of water, could be a major
pollution-free fuel provided problems of
storage and handling can be overcome (2).
Ruthenium Complexes for the
Photolysis of Water
The cleavage of water into gaseous hydrogen
and oxygen requires approximately 70 kcals/
mol (3). Light quanta in the visible region
of the electromagnetic spectrum (400 to 700
m)have low energies, Figure I, and therefore
to achieve water photolysis in this region a
118
potential for this energy-rich species ( 6 )
indicates that it should reduce water, as
protons, to hydrogen over a wide pH range.
While this is a thermodynamically favourable
reaction, in practice no reduction is observed
and the continuing luminescence of the complex in aqueous solution suggests that the
reduction reaction is too slow to compete with
excited state decay. The reduction may also
be inhibited by back electron transfer processes which regenerate the starting materials.
If the reduction reaction
(i)
+
z[*RuL3I2+ 2H++ Hz
+ z[Ru1*LJ3+
where [*RuLJ2+is the excited state of the
complex, could be achieved, then the previously reported oxidation of hydroxide ion (7)
step-wise process involving more than one
photon is necessary. Moreover, since water
is transparent to visible light a photocatalyst is required to absorb the incident
light and transfer its energy to water molecules via redox reactions.
Irradiation of the tris(2,2’-bipyridyl)ruthenium(I1) cation, [Ru(bipy)J2+, with visible
light results in the promotion of an electron
from a molecular orbital of mainly metal
character to one of mainly ligand character (4),
Figure 2. This charge transfer excited state
can act as both an electron donor or electron
acceptor ( 5 ) , and examination of the reduction
Platinum Metals Rev., 1978, 22, (4)
(ii) 2[Ru1I1L3I3+t 2OH- +
2[Ru”L3Iz+
+ H,O 4-4 0 %
could occur. This would produce oxygen
and regenerate the original ruthenium complex. The overall reaction would thus be the
cyclic catalytic photolysis of water.
In 1976 Whitten reported the catalytic
photocleavage of water by ruthenium complexes containing bipyridyl ligands bearing
long chain organic substituents (8). The
complexes initially reported to be active are
shown in Figure 3 and luminesce in air
when illuminated with visible light. Monolayer assemblies of these complexes on glass
119
slides immersed in water and irradiated with
a medium pressure mercury lamp or with
sunlight no longer luminesced and were
reported to produce gaseous hydrogen and
oxygen. More than 103molecules of gas per
molecule of ruthenium complex were produced and 30 cm2of the monolayer produced
0.5 cm3 of gas every 24 hours over a two- to
three-week period. The observed quantum
yield, which is the proportion of excited state
species which react to produce hydrogen, was
about 10per cent. This was higher than that
Platinum Metals Rev., 1978, 22, (4)
observed in photosynthesis in most plants and
indicated a relatively efficient process. More
recent experiments carried out by Whitten (9)
and by other research groups have failed to
reproduce the original observation of water
photolysis. These later experiments have
employed highly purified complexes whereas
the original material successfully used to
photocleave water was subsequently shown
to contain about 3 per cent of impurities. Two
major contaminants have been identified
tentatively as the complexes shown in Figure
4. Assemblies of the highly purified complexes were rapidly degraded upon illumination of the assembly in water with up to go
per cent of the complex being lost from the
glass support within 24 hours. The ruthenium
was subsequently identified as being present
in the water as hydrolysed derivatives of the
original complex, for example the dicarboxylic acid coxlplex (C) in Figure 4. Recent
work has also shown (9, KO) that the packing
of the complex molecules in the assembly is
extremely sensitive to impurities, with the
highly purified complexes having much
higher molecular areas than impure samples.
The spectral properties of pure complex (A),
Figure 3, in solution are similar to those of
[Ru(bipy),lzf (11) but with both the absorption and luminescence maxima occurring at
longer wavelengths, as shown in the Table.
However, the spectral characteristics of complex (A) in monolayer assemblies are considerably different from those in solution.
Samples of (A) of varying purity all exhibited
absorption maxima at higher energy than in
solution and at constant wavelength irrespective of sample purity, while strong variations were observed in both the position and
intensity of the emission maxima. These
spectral differences suggest strong co-operat h e interactions between molecules in the
assemblies and demonstrate the influence of
impurities upon the electronic properties of
the assemblies.
The free radicals H. and 'OH are likely
intermediates in the catalytic photocleavage
reported by Whitten and for the reaction to
120
Spectral Absorption and Emission Maxima for Ruthenium Complexes
Complex
Medium
Absorption, nm
40
450 (11)
452 (4)
solid
CHCI,
monolayer
assembly
I
468 (9)
425 (9)
480 (sh)
all samples
Emission, nm
582 (11)
-580 (4)
660 (9)
645-670 (9)
depending
on purity
The numbers in parenthesis relate to the references, sh-shoulder.
proceed both the recombination of these
radicals to form water:
H. 1 .OH tH,O
and back electron transfer reactions of the type
Ru"'
+ 'H + Ru"
-t H+
must be inhibited. The experimental work to
date suggests that these conditions may have
been met only in the monolayer assembly used
by Whitten. This in turn implies that the
original observation was strongly influenced
by the impurities present in the complex or
by a particular monolayer structure which
has not been reproduced in subsequent
experiments. It is likely that both factors
are important, and possible that a particular
impurity determines the formation of a
particular assembly structure. For example,
impurities containing hydrogen bonding
moieties such as the carboxylic acid groups
in complex (C), Figure 4, could modify the
structure of the monolayer assembly and lead
Platinum Metals Rev., 1978, 22, (4)
to stronger co-operative interactions between
molecules. Such an effect could explain the
disparity in the molecular areas in monolayers of purified and impure complexes.
Unfortunately, many of the important
questions relating to the mechanism of the
photolysis reaction, such as the function of
the long tail substituents and the influence of
molecular packing on the stability and cooperative interactions within the monolayer
assemblies must remain the subject of speculation until the original photolysis reaction has
been successfully reproduced.
The original observation is an important
one because it demonstrates that under the
right conditions the photolysis of water can be
achieved with ruthenium complexes. On the
other hand, it seems unlikely that monolayer
systems will ever form the basis of a practical
process of solar energy conversion because of
the large surface areas which would need to
be covered with films having reproducible
121
of these modes have been described recently.
In an example of the complex acting as an
electron donor two half-cells both containing
[Ru(bipy),12+,Fe3+and H,O+ are connected
through an external circuit with one half-cell
illuminated while the other is kept dark
(6), Figure 5. In the illuminated half-cell
photo-induced electron transfer generates
[Ru(bipy),]8+ and Fez+:
[Ru"(bipyj,lJ-
+
+ hv-
[*Ru(bipy),j2+ Fe3-
properties. A more promising area involves
research on dissolved ruthenium complexes
(12) in an attempt to find a homogeneous
photolysis reaction which would be more
amenable to scaling-up into a practical
energy conversion process.
[*Ru(bipyj,12'
[RuI1I(bipy),l3++Fe'+.
The changed composition of the illuminated half-cell results in a change in its potential
relative to the dark half-cell and electrons flow
through the external circuit in the direction
determined by the dominant couple, from the
dark to the illuminated half-cell. The reversibility of the cell is ensured by mass transfer
of Fez&to the dark half-cell where it reacts
with anodically produced [Ru"'(bipy),] 3+
to regenerate [R~~'(bipy),]~+
and Fe3-.
Photogalvanic Cells Containing
Ruthenium Complex Electrolytes
Illumination of dilute aqueous solutions of
many redox couples leads to a change in the
potential of the couple (13). Photogalvanic
cells based upon this principle have been
known for several decades, for example ironbut have not been
thionine photocells (14)~
seriously considered as a practical means of
converting light into electricity because of
their low efficiencies. However, the efficiencies of such cells may now be considerably
improved by the recent developments in thin
layer devices and semiconductor electrodes.
Simultaneously, new cells without the inherent
disadvantages of some of the older systems are
now emerging.
As previously stated the excited state of
[Ru(bipy),lz' can function as both an electron
donor or electron acceptor, and photogalvanic
cells in which the complex functions in either
Platinum Metals Rev., 1978, 22, (4)
+
122
Figure 6 shows the reactions proposed in trode would flow through the external circuit
this cell. The observed open-circuit photo- to reduce water or hydrogen ions at the metal
potential depends upon the incident light electrode to dihydrogen:
zH- ae--H,
intensity and on the acid used in the cell but
The
original
ruthenium complex would be
under typical conditions potentials of 100 to
180 mV have been observed. The long term regenerated in reaction (ii), page 119. Many
stability of the system is relatively good since factors may be critical to the realisation of
only approximately 20 per cent of the such a cell. Choice of electrode material may
ruthenium complex is degraded after 6 days be one important factor since not only would
on a cycle of exposure of the cell to a 500W it be desirable to use a cathode whose overtungsten lamp for 10 seconds in every potential for the production of hydrogen is
minute. Only a very small catalytic amount small, but the semiconductor electrode must
of ruthenium complex is required and the be chosen so that the excited electron has
small long term loss could easily be rectified sufficient energy to reach its conduction band.
Although a cell containing a serniconby addition of further ruthenium complex
without the cost of the complex becoming a ducting tin oxide electrode and based on
[Ru(bipy),lL+has been reported (16), the cell
prohibitive factor.
In the absence of an efficient quencher, mechanism here is probably similar to that
such as Fe3+, the excited state ruthenium proposed for the Ru"/FeJl cell previously
complex can function as an electron acceptor described with methyl viologen taking the
towards a gold electrode (15). The reactions place of Fe3' as the homogeneous quencher.
occurring in this cell are:
Open circuit currents of 160pA may be
observed
in this cell which functions to
[*Ru(bipy)J2+ e + [Ru'(bipy),l+
reduce
atmospheric
oxygen to hydrogen
at the illuminated cathode
peroxide, but the observed quantum yield is
[Rul(bipy)J* + [Ru11(bipy),]2t 4 e
again low at about 0.5 per cent.
a t the dark anode.
Electron transfer from an excited species
This cell is sensitive to dissolved molecular such as [*Ru(bipy)J2-' to an electrode will be
oxygen and is best operated under an inert sensitive to many effects including the ionic
atmosphere. Current yields are very low, only and molecular environment of the complex,
z to 16kA, because decay of the excited com- its excited state lifetime and its proximity to
plex to its ground state is about 105 times the electrode surface. An active area of
more probable than electron transfer at the current research is the study of the chemical
gold electrode.
linking of such reactive species to electrodes
A photocell based on [Ru(bipy),]*+which to facilitate electrochemical reactions (17).
has been proposed (7) but not yet reported Preliminary evidence suggests that ruthenium
contains a metal electrode and an n-type bipyridyl complexes attached to an electrode
semiconductor electrode immersed in a solu- surface could provide a completely reversible
tion of the complex buffered at pH 9 and electroactive system and studies of this sort
connected via an external circuit. It was pro- could give a valuable impetus to the search
posed that illumination at the semiconductor for more efficient photoelectrochemicalenergy
electrode would produce [*Ru(bipy),]*. which, conversion systems.
under favourable conditions, would inject an
Photocells based on [Ru(bipy),J2+ have
electron into the conduction band of this several advantages (6) over some of the better
electrode leaving [R~'~'(bipy),]
3+ which could
known photogalvanic systems such as ironthen oxidise hydroxide ions to oxygen as thionine photocells. These are:
previously described.
(a) [*Ru(bipy),]?+is very stable with respect
The electron at the semiconductor electo reaction with oxygen in the presence of
+
Platinum Metals Rev., 1978, 22, (4)
123
an efficient quencher, as for example Fe3+
(b) The maximum absorption of the complex
[Ru(bipy),12+is close to the maximum intensity peak in the solar spectrum, as
shown in Figure I .
(c) No reactive intermediates giving rise to
undesirable side effects, which is a major
drawback with the iron-thionine system,
are involved in the kinetics.
Photochemical cells based on [Ru(bipy)Ja+
have emerged only very recently and no doubt
the inefficient design of the present cells could
be drastically improved. At thc same time
preliminary experiments with different ruthenium complexes indicate that the quantum
yield may also be improved. It is unrealistic
to expect such cells to generate large amounts
of power but it is not inconceivable that cells
of this sort connected in series could have
applications in the electrolysis of water to
hydrogen.
have relevance in solar energy conversion
schemes. In this complex, orbital interaction
between the two metal atoms results in intense
electronic absorption in the visible spectrum.
When the blue solution of the complex in
12M hydrochloric acid is irradiated with
light at 546 nm the yellow complex
[Rh,(bridge),Cl,]*+ and gaseous hydrogen are
produced in a non-cyclic reaction. The
mechanism has not yet been fully established
but the available evidence points to an
initial protonation of the dimer:
Bridged Rhodium Complexes for
Hydrogen Production
In concentrated hydrochloric acid the
quantum yield for this reaction is very low,
only about 0.5 per cent, but this is increased
considerably to about 4.4 per cent in concentrated hydrobromic acid, and this observation
may provide a clue to methods of further
increasing the quantum yield. The excited
An unusual dimeric complex of rhodium
containing four bridging r,3-diisocyanopropane ligands and having a windmill-like
structure, Figure 7 , has been reported recently
(IS) to possess novel properties which may
Platinum Metals Rev., 1978, 22, (4)
[Rh,(bridge),12+t HI
+ C1 +
[Rh,(bridge)4H]3+C1-
where the complex formed i s a proton adduct
rather than a hydrido complex. This is followed by a second electron transfer reaction
which is light induced:
[Rh,(bridge),HI3+ i H,O+
hv546nm+[Rh,(bridge)4C1,12t
r2MHCl
124
A
zC1
+ H z + H,O.
the very low quantum yields presently found
in both the rhodium complex and semiconductor cell reactions.
state species has been formulated as
[Cl-. . . R h 3 / z . . . Rh3/2. . . .HI2+*and it
has been suggested that the o-binding interactions along this internuclear axis are
stronger than in the ground state and therefore favour the production of hydrogen over
back-transfer to the Rh(1) complex. I n
practice the back reaction :
I atmosphere H,
H, “%(bridge)4C1212+
3oIK
i
Conclusions
+
[Rh,(bridge),HI3+C1is indeed slow and takes several days to go to
completion.
This type of homogeneous reaction is a very
attractive proposition for future energy conversion processes provided it can be made
cyclic with regeneration of the original Rh(1)
complex. Attempts to achieve this end by
methods such as coupling the rhodium system
to a semiconductor cell for the simultaneous
production of oxygen and regeneration of the
active Rh(1) complex by reduction are currently under investigation (12). Such work is
still at a very early stage and some formidable
problems will need to be solved before such a
combination could be considered as a practical
method for the production of hydrogen, Not
the least of these problems will be to increase
Research into solar energy conversion processes nowadays includes not only work in
established areas such as thermochemical,
photovoltaic and bio-photolytic methods but
also in novel areas such as energy conversion
via photoassisted valence isomerisation reacThe area involving cotions (19, 20).
ordination compounds of the platinum group
metals forms only a small but apparently
rapidly growing part of the overall picture.
Many of the different methods using platinum
group metal complexes which have been
described are promising, but all are presently
at an embryonic stage and require considerable research and development before they
could become practical systems for solar
energy conversion. Nature has been using
the sun as a direct energy source for photosynthesis since life first evolved on earth.
Present research on solar energy conversion
attempts to emulate nature, and is therefore
one of the most challenging projects for the
modern scientist.
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